Track and hold circuits, also referred to as sample and hold circuits, provide samples of an analog or digital input signal and maintain the value of each sample for a predetermined time interval. Typical applications for such circuits are in analog-to-digital and digital-to-analog converters.
In general, the track and hold circuit comprises an electronic switch and a signal storing device. During each tracking interval, the switch is closed in response to a track control signal and the input signal is coupled to the signal storing device. During each holding interval, the switch is opened in response to a hold control signal, thereby isolating the signal storing device.
A problem with track and hold circuits is that each transition from track to hold deposits or removes an electronic charge from the stored signal. As a result, a track-to-hold pedestal or offset is produced which generally varies from sample to sample. In many telecommunications applications, the resulting sampling error is unacceptable.
Other problems in track and hold circuits are feedthrough and "droop" which produce variations in the signal stored during the holding interval. Feedthrough is input signal dependent and is related to the time rate of change of the input signal during the hold interval. Droop, on the other hand, is input signal independent and may be related to anomalies in the track and hold circuit components.
A variety of prior art techniques have been utilized to alleviate the problems of track-to-hold offsets, feedthrough and droop. Combining these techniques, however, is expensive to implement and/or reduces the operating speed of the track and hold circuit. Consequently, a low cost track and hold circuit that provides precise samples and is operable at high speeds would be desirable.